1. Field of Invention
The invention relates to a piezoelectric actuator and an ink-jet head using a piezoelectric actuator, and more particularly relates to a flat piezoelectric actuator that is less prone to warpage or waving, and to an ink-jet head using such a piezoelectric actuator.
2. Description of Related Art
A piezoelectric actuator converts electric energy to a mechanical displacement to thereby cause movement of a mechanism. Such a piezoelectric actuator is assembled with various parts into an end product, such as an ink-jet head for an ink-jet printer.
As disclosed in U.S. Pat. No. 5,402,159, a conventional on-demand type piezoelectric ink-jet head includes a cavity plate and a piezoelectric actuator. The cavity plate has a plurality of nozzles and pressure chambers, each associated with one of the nozzles. The piezoelectric actuator is formed by laminating piezoelectric sheets, each of which is sandwiched by individual flat electrodes provided individually for the pressure chambers and a common electrode provided commonly for the pressure chambers. The piezoelectric actuator is stacked on the cavity plate such that the individual electrodes are aligned with the corresponding pressure chambers in the cavity plate.
In the piezoelectric actuator of the ink-jet head structured as described above, through-holes are formed in the piezoelectric sheets, as proposed in Japanese Patent Publication No. 7-96301, to electrically connect the individual electrodes and the common electrodes in the piezoelectric sheet laminating direction. Through-holes are formed to penetrate the piezoelectric sheets in the thickness direction at positions corresponding to the individual electrodes and the common electrodes, and are filled with a conductive paste.
Usually, green sheets formed with electrodes and through-holes are laminated and sintered into a single body. Then, the sintered body undergoes polarization to gain a piezoelectric property, and is produced as a piezoelectric actuator.
However, the green sheets formed with thorough-holes shrink locally during the sintering. As a result, warps or waves are produced in the sintered body (piezoelectric actuator). As the through-hole diameter becomes large, warps or waves become serious and thus flatness of the resultant piezoelectric actuator is degraded.
If a warped or wavy piezoelectric actuator is used in an ink-jet head, a gap is created between the piezoelectric actuator and a cavity plate when they are bonded. Such a bonding failure results in ink leaks and improper ink ejection.
Other than use in an ink-jet head, a piezoelectric actuator is generally placed over or incorporated into other parts. When a piezoelectric actuator is not flat, the piezoelectric actuator is not properly joined or bonded to a mating part, and a resultant end product only delivers limited performance.
The invention addresses the forgoing problems and provides a piezoelectric actuator that is less prone to warpage or waving and flat enough to offer high performance. The invention also provides an ink-jet head using such a piezoelectric actuator.
According to one aspect of the invention, a piezoelectric actuator includes first piezoelectric sheets and second piezoelectric sheets. Each of the first piezoelectric sheets has inner individual electrodes, and each of the second piezoelectric sheets has an inner common electrode. The first and second piezoelectric sheets are laminated alternately. Though-holes are formed in the first and second piezoelectric sheets and coated with a conductive material such that the inner individual electrodes are electrically connected, at their end portions, with one another in a laminating direction of the first and second piezoelectric sheets and the common electrodes are electrically connected, at their at least one end portion, with one another in the laminating direction. The maximum diameter of each of the through-holes is not less than about 20 xcexcm and not more than about 200 xcexcm.
When the maximum diameter of each of the through-holes is within this range, warps and waves that may be produced during the manufacturing process of the piezoelectric actuator are reduced and a flat piezoelectric actuator is obtained.
According to another aspect of the invention, the piezoelectric actuator is stacked on a cavity plate formed with pressure chambers and nozzles, each communicating with a corresponding one of the pressure chambers. The first piezoelectric sheets extend across the pressure chambers, and the inner individual electrodes on each of the first piezoelectric sheets are each aligned with a corresponding one of the pressure chambers. The second piezoelectric sheets extend across the pressure chambers, and the inner common electrode on each of the second piezoelectric sheets extends across the pressure chambers.
When the piezoelectric actuator, that is flat, is bonded to the cavity plate, no gap is created therebetween, and no ink leaks result.